<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <link rel="stylesheet" href="./demo.css" />
    <script src="./js.js"></script>
</head>

<body>
    <div class="box">
        <div class="ball ball1">1</div>
        <div class="ball ball2">2</div>
        <div class="ball ball3">3</div>
        <div class="ball ball4">4</div>
    </div>
</body>

</html>